Heat pipes are utilized to transfer heat away from a source of heat to a heat sink. For example, heat pipes are useful in the control of the temperature of electronic equipment. Heat pipes may take various forms other than a tube or pipe from which common form the device takes its name. A typical heat pipe is a closed chamber containing a small amount of working fluid. The interior of the chamber contains a capillary wick which comprises a cluster of fine wire, a plurality of fins extruded on its interior walls, etc. The free space within the chamber contains only vapor of the working fluid at the saturation pressure of the fluid at the ambient temperature of the environment in which the heat pipe is to operate.
If the temperature of the heat source and hence one end of the heat pipe should rise above the ambient temperature, the liquid in that end vaporizes. The resulting vapor is at a higher pressure and hence will travel toward the lower pressure, cooler end of the heat pipe where it condenses. As the vapor condenses it gives up the latent heat acquired during vaporization. The capillary wick within the heat pipe is dimensioned to transport the condensate back to the heat source area by capillary action where the cycle is repeated.
In many heat pipe applications it is advantageous to be able to control the thermal conductance of the heat pipe. For example, electronic equipment may generate more heat during certain phases of operation and, therefore, require the removal of more heat to maintain a fixed temperature. Alternately, it may be desirable to operate particular equipment at a variety of temperatures. It is possible to accommodate both of these situations by removing heat from the equipment through a controllably variable conductance heat pipe.
Prior art heat pipes have controlled the fluid flow within the heat pipe by controllably blocking the fluid path between the evaporator section and the condenser section.